Arc devices such as plasma guns are utilized for such purposes as themal spraying which involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface to be coated. In typical plasma systems an electric arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electric arc and is excited thereby to temperature of up to 15,000 degrees Centigrade. The plasma of at least partially ionized gas issuing from the nozzle resembles an open oxy-acetylene flame.
In copending patent application Ser. No. 021,958 filed Mar. 5, 1987, assigned to the same assignee as the present application, a plasma generating system comprises a plasma gun including a hollow cylindrical anode member, a hollow cylindrical intermediate member electrically isolated from and juxtaposed coaxially with the anode member to form a plasma-forming gas passage through the intermediate member and the anode member, and an axially movable cathode member. An electric motor or pneumatic piston responsive to a measuement of arc voltage continually adjusts the axial position of the cathode tip relative to the anode nozzle so as to maintain a predetermined arc voltage.
This system with adjustment of the cathode according to voltage has proven itself to provide a substantial improvement in arc gun performance. The electric motor and pneumatic piston arrangements disclosed in the copending patent application, are operatively very efficient. However, they are somewhat bulky, heavy and complex or require separate utility (compressed air).
U.S. Pat. No. 3,242,305 discloses a retract starting plasma torch in which starting of the arc is accomplished by a spring urging an electrode against the nozzle. Retraction to a fixed operating position is effected by the fluid pressure of the cooling water acting against the spring when the arc is started.
In view of the foregoing an object of the present invention is to provide an improved arc device with an adjustable cathode position relative to the anode.
A further object is to provide a novel cathode adjustment mechanism utilizing the cooling fluid for the arc device.